In using aluminum plates as supports for printing plates, they are usually subjected to a treatment for roughening their surfaces in order to ensure good intimate adhesion between the aluminum plate and a light-sensitive film to be provided thereon and improve water retention in non-image areas. This surface-roughening treatment is called graining, and includes mechanical graining such as ball graining, sand blast graining or brush graining, electrochemical graining which is also called electrolytic polishing, and chemical etching which is called chemical graining. These conventional graining processes possess advantages and disadvantages. In general, problems with the mechanical graining process include scuff marks, stains, residue of an abrasive used, etc. The electrochemical graining process makes it possible to change the depth of the graining as well as the form of grains by controlling the quantity of electricity. However, it requires a large quantity of electricity and a long time for creating grains suited for printing plates, leading to high production costs.
On the other hand, the chemical graining process grains an aluminum or aluminum alloy plate by a chemical etching reaction using an acid or alkali etchant, and hence it is simple and suited for continuously treating aluminum or aluminum alloy strips, and is particularly advantageous for industrially producing plates having been treated on both sides. However, it has so far been difficult to produce high quality printing plates using commercially available aluminum or aluminum alloy. Because, conventional chemical etching processes have difficulty in forming a surface having enough surface roughness and uniform pit pattern (wherein etching pits have a uniform diameter and a uniform depth) to give sufficient printing durability and staining resistance required for printing plates.